Cook pasta according to package. Slice mushrooms & onions. Slice steak in the thinnest possible strips. (This will be easier if you put the steak in the freezer for 30 minutes before starting.) Put oil in a large, heavy skillet, and sauté onions until they soften, 2-3 minutes. Add mushrooms and sauté another 3-4 minutes, until onions are very soft and mushrooms are cooked. Remove from pan and reserve. Put steak in hot pan and cook quickly, stirring all the while. Remove steak from pan when it is barely cooked – some strips may still be faintly pink. Turn heat to high and add beef broth. Let cook 5 minutes. Turn heat to low, add reserved vegetables and meat and cook 1 minute. Push food to the sides of pan and stir cornstarch/broth mixture into the middle in a thin stream. Stir pan and cook until sauce thickens slightly, about 1 minute. Turn off heat and add sour cream. Serve over pasta.